1. Field of the Invention
The present invention relates to a road condition monitoring system, and more particularly to the system for determining a coefficient of friction of the road surface, which is applicable to an anti-skid control system for controlling braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability or the controllability is influenced detrimentally by the road surface condition, when road wheels are locked in abrupt braking operation. In order to prevent the road wheels from being locked, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding a hydraulic braking pressure supplied to a wheel brake cylinder, and which is also called as an anti-locking control system. In view of the fact that when the hydraulic braking pressure supplied to the wheel brake cylinder is increased, the rotational speed of the road wheel is rapidly reduced immediately before the coefficient of friction of the road surface relative to the road wheel reaches its maximum value, the anti-skid control system controls the wheel cylinder pressure according to the deceleration of the vehicle in order that a slip rate of the road wheel results in around 20%, that is, the maximum coefficient of friction is obtained.
The above-described coefficient of friction of the road surface varies according to the type of the road wheel, the condition of the road surface and other factors. Especially, it is much influenced by the condition of the road surface, such as dry or wet road surface. Therefore, the coefficient of friction (hereinafter, simply referred to as CF) of the road surface is a very important factor for the anti-skid control, and it is necessary to control the braking force in response to the CF of the road surface in order to raise the efficiency of the braking operation with the stability of the vehicle maintained. Although it is impossible to detect directly the CF of the road surface on the running vehicle, it is possible to estimate the CF of the road surface from the deceleration of the vehicle in its braking operation, so that an acceleration sensor is employed as disclosed in Japanese Patent Laid-open Publication No. 54-74983. Accordingly, the output of the acceleration sensor is provided for estimating the vehicle speed, or changing a speed or quantity of the brake fluid during the control of increase or decrease of the hydraulic braking pressure. In terms of the acceleration mentioned above, the deceleration is included therein, and when the terminology of the acceleration is employed, the deceleration will be included therein unless otherwise defined, hereinafter.
As for the acceleration sensor, various sensors are known such as a mercury switch, or those using a moving-coil, a pendulum, piezoelectric or photoelectric elements. However, when these are installed on a vehicle to detect the deceleration thereof, the output of the acceleration sensor may not correspond to the deceleration of the vehicle due to a vehicle vibration, for example, Therefore, in the case where the output of the acceleration sensor indicates a high acceleration (hereinafter referred to as High-G) or a low acceleration (hereinafter referred to as Low-G), if the CF of the road surface is determined corresponding directly to the above outputs, a hydraulic braking pressure will be increased to cause the lock of the road wheel when the vehicle is running on a road of a low coefficient of friction (hereinafter referred to as Low-CF), or a hydraulic braking pressure will be so much decreased to increase the braking distance when the vehicle is running on a road of a high coefficient of friction (hereinafter referred to as High-CF).
In Japanese Patent Laid-open Publication No. 54-74983, it is proposed that a predetermined delay of time is to be provided in the anti-skid control for changing the High-CF mode thereof to the Low-CF mode thereof, whereas no delay of time is to be provided in the control for changing the Low-CF mode to the High-CF mode. And, it is proposed in Japanese Utility model Laid-open Publication No. 64-21056 that the anti-skid control is changed to the control for the road of High-CF or that for the road of Low-CF in case of detecting the High-G or Low-G respectively.
However, either system disclosed in the above publications will cause the lock of the road wheel in braking operation on a rough road with continuing uneven ground of Low-CF such as that of a pressed snow road. This is because, according to the art disclosed in the former publication, when intermittent High-G inputs are caused by the uneven road with a period shorter than the delay time in changing from the High-CF mode to the Low-CF mode, the anti-skid control is fixed to that in the High-CF mode, also because, according to the art disclosed in the latter publication, the High-CF mode is selected for the rough road irrespective of the actual CF of the road surface.
FIG. 9 shows the anti-skid control operation of the above prior art when the acceleration detected by the acceleration sensor on the rough Low-CF road varies as shown in (A). In the case where the detected acceleration vibrates as shown in (A), if the period of High-G is shorter than the delay time in changing from the High-CF mode to the Low-CF mode, the result of determination of the road condition is fixed to the High-CF mode as shown in (B). Or, it is fixed to the High-CF mode due to the rough road. Consequently, in the estimated vehicle speed set in accordance with the wheel speed and the CF of the road surface, the estimated vehicle speed Vso calculated according to the High-CF mode is set to be lower than the actual estimated vehicle speed to be set at the time of the Low-CF mode as shown in a phantom line in (C), so that the difference between those estimated vehicle speeds becomes large when the wheel speed Vw is decreased, and resultantly the lock of the wheel is caused as shown at the right side of (C).